Autofocus in a digital camera, typically used for capturing still images and a series of still images for video segments, often uses a through the lens autofocus system based on a contrast analysis of a series of say 5-20 or more subsampled autofocus images taken with a moveable lens in different positions. Subsequently, the autofocus images are analyzed for contract and the moveable lens position that delivers the image with the highest contrast is deemed the best focus condition. The moveable lens is then returned to the position that delivered the highest contrast or an interpolated position between the at least two autofocus images before the still image is captured. Although this approach does deliver an accurate focus condition, it is slow due to the many autofocus images that must be captured and analyzed.
During video capture, the autofocus images are derived from the same series of still images or frames that compose the video segment. Consequently, the process of autofocusing causes the 5-20 or more out of focus frames to be produced in the video each time the scene changes. As a result, during video capture with pan movements of the camera where the scene changes continuously, large portions of the video segment are actually out of focus. Ideally, the autofocus system would be faster when capturing video as well as still images and, in the case of video capture, each frame would be focused so that the number of out of focus frames is reduced. This is especially important in enabling images from video segments to be printed or used in other fashions.
For autofocus to become fast enough to enable frame by frame autofocus at the typical video frame rate of 30 frames/second, several improvements are needed throughout the autofocus system. First, a focus measurement system is needed that can provide a measure of focus quality that can be completed within 1/30 second or faster. The focus measurement must provide enough information to accurately guide the movement of the moveable lens in terms of distance and direction to achieve the desired focus quality within 1/30 second as well. Second, the moveable lens movement control system must be fast enough to reposition the moveable lens within 1/30 sec. Third, the entire autofocus system, composed of the focus measurement system and the moveable lens movement control system, must be accurate over the length of time that the video will be captured, which is typically on the order of minutes but could be longer.
There are several types of focus measurement systems in the prior art that are sufficiently fast to meet the 1/30 sec requirement: a dual lens rangefinder module, a split color filter system, a split aperture device. All of these focus measurement systems require special modifications to work in the video environment to deliver frame by frame autofocus.
Dual lens rangefinder modules can be purchased from Fuji Electric in several models such as the FM6260W. The Fuji Electric rangefinder module is described in U.S. Pat. No. 4,606,630 to Haruki. Dual lens rangefinder modules contain two lenses that are separated by a distance along with two matching sensor areas to enable matched pairs of low resolution images to be captured. The matched pairs of low resolution images are then analyzed for correlation between the two images to determine the offset between the two images caused by the separation between the two lenses. The offset information is then used along with the lens separation distance to calculate the distance to the scene by triangulation. The calculated distance to the scene is used to guide the positioning of the moveable lens to produce the best image quality based on a calibration curve established between the distance to the scene as measured by the dual lens rangefinder module and a series of best focused images as produced by the through the lens autofocus system. The response time of the Fuji FM6260W modules is advertised as 0.004 sec in high sensitivity mode, which is well within the 1/30 sec required for video autofocus. The accuracy of dual lens rangefinder modules however, are typically influenced by changes in the environmental conditions such as changes in the temperature or humidity. Typically, these dual lens rangefinder modules are not used independently for autofocus in digital cameras but instead are used as a rough focus adjustment that is supplemented by a through the lens contrast based autofocus system. The problem with the dual lens rangefinder modules is that the calibration between the dual lens rangefinder module and the moveable lens position is not stable within the normal operating environment for digital cameras. Environmental conditions such as changes in temperature and humidity can cause the calculated distance to the scene produced by the dual lens rangefinder module to change by over 10%. In addition, the measured position of the moveable lens in the moveable lens control system is prone to environmentally induced changes as well.
Both a split color filter system and a split aperture device use a split aperture in the lens system to create images that can be interpreted for focus information. The split aperture creates at least two optical paths for the light passing through the lens to create at least two autofocus images at the sensor. By splitting the optical path at the aperture of the lens system, each of the at least two optical paths creates a full image without shading but reduced light intensity at the image sensor from the split aperture.
In the case of the split color filter system, a split color filter is inserted into the optical path of the lens at the aperture position. The split color filter is constructed so that the filter area is divided into at least two different areas with different colors in the different areas. Two autofocus images are then captured simultaneously as a first autofocus image overlaid on top of a second autofocus image, but since the first and the second autofocus images are different colors they can be differentiated in the overlaid image in areas where they do not overlap. In the case of the split aperture device, the aperture is sequentially partially blocked over at least two different portions of the aperture, to create the at least two optical paths. Because the at least two optical paths in the split aperture device do not have different colors, the split aperture device requires that autofocus images are captured for each partially blocked condition resulting in at least two autofocus images. In both cases, the difference between the at least two optical paths causes the autofocus images to be displaced laterally in proportion to the degree of defocus and direction of defocus for an object in the image.
A split color filter system for autofocus was described in Keiichi JP 2001-174496. In this case, a color filter composed of two different colors on opposing sides of the aperture creates two overlaid images of different colors (typically blue and red) on the sensor. Any defocus present in the image creates an offset between the two images which then shows up as color fringes on either side of the object in the image. Movement of the focusing lens reduces or enlarges the color fringes in the image depending on the distance from focus. When the image is well focused, the color fringes disappear. Defocus inside of the focal point causes the fringes to be one color on one side and the other color on the other side of the object in the image. Defocus outside of the focal plane results in the colors of the color fringes being reversed. Consequently, with this approach, one image taken with the split color filter delivers an autofocus image that can be analyzed to determine the degree of defocus and the direction of defocus. Keiichi does not use a split color filter system for autofocus during video and in addition, the split color filter used by Keiichi is fixed in the optical path so that substantial light losses are incurred from the split color filter in the optical path during capture of the video images. In general, the addition of a split color filter to the optical path introduces light losses to the optical system which tend to slow down the autofocus system and make autofocusing under low light conditions difficult.
Split aperture devices for autofocus were described by Kurahashi in JP 1997-184973, Horikawa in U.S. Pat. No. 4,631,394, and Wolbarsht in U.S. Pat. No. 4,201,456. In these disclosures, the aperture is alternately partially obstructed thereby creating multiple optical paths. The autofocus images that are captured for each of the alternate multiple optical paths are shifted laterally when compared one to another in proportion to the distance from focus. In the case where the optics are focused, the multiple images are not shifted laterally when compared one to another as the aperture is alternately partially obstructed. A set of images are collected for at least two of the optical paths in which the aperture is partially blocked. A comparison of the image set enables the lateral offsets between images to be identified and the related distance from focus to be calculated. Consequently, with one set of images with at least two different partial obscurations of the aperture, the lateral difference between the at least two images identifies the degree of defocus and the direction of defocus. However, Kurahashi, Horikawa, and Wolbarsht do not use a split aperture device for fast autofocus of stills or for autofocus during video capture.
Therefore, a need exists for autofocus systems to be modified for use during video capture to provide a frame by frame autofocus.